This is a proposal to investigate the control of cell-type-specific gene expression in mammalian cells. Using highly differentiated rat hepatoma cells growing in vitro as a model system, an integrated genetic/molecular biological approach will be applied to study the regulation of expression of liver-specific genes. Genetic analyses will be performed with the use of a novel monochromosomal hybrid panel (MHP). This panel consists of twenty nine hepatoma clones, each retaining a single, specific mouse fibroblast chromosome. Studies of the type have defined a novel class of mammalian genes, the Tissue-specific extinguisher (Tse) loci, that regulate expression of unlinked structural genes in trans. The goals of this project will be to further define the Tse loci and their phenotypic effects, to isolate a particular locus (Tse-2) by molecular cloning to study its structure and function in detail, and to define the cis sequences in tissue-specific structural genes that are required for Tse control. Ultimately, we hope to precisely determine the role of Tse regulatory loci in differentiation and development, and to understand in molecular detail the mechanisms by which they function. These studies should provide important insights into perturbations of cellular regulatory controls that underlie abnormal developmental situations and neoplasia.